There are many economic theories explaining the correlation between country income indicators (GDP per capita, country income classification, etc.) and health-related indicators (life expectancy, fertility rate, infant mortaility rate, etc.). At the macroeconomic level are the Preston curve and Endogenous Growth theory. First, the Preston Curve (1975) illustrates the positive but concave relationship between GDP per capita and life expectancy, showing that richer countries generally live longer, but with diminishing returns: income gains matter more at lower income levels, while at higher levels, other factors like medical advances cause bigger leaps in longevity. It was also theorized and supported by reseach that the entire curve moves outward (meaning longer life expectancies for the same income), due to major improvements in public health, education and nutrition. Second, the endogenous growth theory (Lucas [1988] being one of the theorists) explains that growth is driven by human capital accumulation and that health improvements increase labor supply and innovation and increases returns to education, and in the long run, sustains economic growth.

At the microeconomic level, Grossman (1972) wrote on the demand for health wherein, in gist, he explains that there is utility derived from being healthy and living longer. Hence, income is spent (considered as investment) in health products and services such as healthcare, medical products and services, nutrition, exercise, and education. Higher income per capita means possible higher allocation to investments in health, hence, raising life expectancy. Finally, also at the household level, Becker (1961) and Schultz (1962), theorists of human capital, explained that health is considered a component of human capital and is considered a driver of productivity and hence, in the long run, a driver of income.